Bleaching fibrous material



Nov. z5, 1947. l R. P. HM 2,431,478

l BLEACHING FIBROUS MATERIAL Filed July 25, 1942 BY '4, f. (g4-@4WATTORNEYQS.

Patented Nov. 25, 1947 UNITED STATES PATENT OFFICE BLEACHING FIBROUSMATERIAL Raymond P. Hill, Wausau, Wis.

Application July 25, 1942, Serial No. 452,329

Claims.

The present invention relates generally to improvements in the art oftreating brous material or the like, and relates more specifically to animproved method of and apparatus for bleaching and purifying cellulosicpulps.

An object of my invention is to provide an improved method of andapparatus for carrying out in a continuous iiow operation the chemicalreaction phase of any process, whether in singleor multiple-stage, forbleaching or purifying cellulosc pulps or the like.

The basic operation in the bleaching of cellulosic pulps for thepreparation of paper making stocks, and in the purification of suchpulps for the preparation of cellulose derivatives, consists in treatinga slurry of the fibrous material and water with a chemical reagent,under predetermined conditions of time, temperature and concentration,and then washing the brous material free from the products of thechemical reaction. Nearly al1 modern systems of bleaching and puricationcomprise a plurality of such steps, Whereby the fibrous material issubjected to treatment by a sequence of chemical reagents adapted toproduce the desired characteritsics of product with the greatestchemical efficiency. Many of these systems employ up to seven or moresuch steps in sequence.

In the previous design of such systems the pulp slurry under treatmentis often erroneously considered as being a substantially homogeneousmass, whereas it is in reality an aggregate of a vast number ofnon-identical fibres possessing unequal or different demand for andreactivity to the various chemical agents employed. This fact imposesthe requirement that the fibre and reagent be not only well mixedinitially, but also periodically re-mixed at suitable intervalsthroughout the reaction cycle. If this requirement is not adequately metand if chemical dosage is based on average demand, some of the bres willnaturally be incompletely treated. Or, if chemical dosage is madesufficient to completely treat the bres having individual maximumdemand, other fibres will obviously be overtreated and degraded, and asubstantial part of the chemical agent will be wastefully consumed. Ihave discovered that in a typical commercial operation of bleaching withcalcium hypochlorite the use of suitable mechanical means to give aperiodic re-distribution and re-mixing of the chemical solution andfibre will effect a saving of ten to twelve percent in the consumptionof chemicals and an improvement of fteen to twenty-,live percent in thepaper-making quality of the product, as compared to an otherwiseidentical operation which depends wholly on initial mixing of the breand chemical. Production of paper making stock having a high degree ofbrightness combined with maximum physical strength, and also thepreparation of purified cellulose for the production of explosives,photographic lm, rayon, and other related products, further requiresthat the factors of chemical dosage, temperature, pulp consistence andreaction time, be controlled and uniformly maintained with a high degreeof precision, in each of the several stages of the bleaching systememployed.

All these requirements have been quite successfully met in commercialoperation by the use of batch systems in which, in each stage of theprocess, a definite quantity of pulp slurry of predeterminedconsistence, together with measured quantities of chemical reagent, areplaced in a reaction vessel having suitable mechanical means forcirculating the pulp mass in a closed circuit with periodic re-mixing ofthe nbre and solution. After processing for a denite period of time atcontrolled temperature each batch of material is discharged from thereaction vessel, and the cycle is repeated with a new batch of material.While this prior batch method of treatment has been found relativelysatisfactory so far as ultimate results are concerned, it requires aplant which i-s complex and costly, and its operation is tedious andexacting. The time consumed in filling and discharging the reactionvessels is non-productive, resulting in the necessity for a very largenumber of such units in a multi-stage system of even moderate capacity,and in complicated and cumbersome equipment for transporting the pulpbetween stages and selectively delivering it to and receiving it fromthe numerous raction vessels. These elements in turn require a heavyinvestment in building structures, power equipment and otherauxiliaries.

It is elementary in theory to overcome these disadvantages by employinga system based on the principle of continuous flow, wherein a constantflow of pulp slurry passes continuously into and out of one or moreretention Vessels whose volume, in relation to the rate of ilow, is suchas to give the desired period of reaction time. Several suchinstallations have heretofore been constructed, employing reactionvessels of various contours and proportions and with various types ofmechanical auxiliaries; but in all cases serious difficulties have beenencountered in operation, and such installations have been foundunsuitable for the higher grades of product. One 0f the most seriousdifliculties encountered has been the impossibility of obtaining uniformtreatment of the material passing through the reaction vessels, becauseof certain inherent characteristics of a suspension of brous material inan aqueous medium. Such a suspension can be treated neither as asubdivided solid nor a viscous liquid, and may be readily conveyed as afiuent slurry only within a narrow range of velocity. Fibres will notremain uniformly dispersed in a iiuid medium, but tend to agglomerate invarious ways according to their basic type, the degree of treatment theyhave received, and other variables. The fibres and fluid tend toseparate, sometimes by precipitation and sometimes by flotation, andtherefore tend to travel at different velocities under the influence ofgravity or propulsion. When it is attempted to pass such a mixture in aconstantly advancing stream through a reaction vessel at the relativelyslow velocity required by processing time, the flow will behave muchlike a natural stream flowing through swampy ground, in that certainlimited areas will pipe or channel at much higher than average velocity,while greater areas will remain sluggish or even coinpletely stagnant.In this manner portions of the pulp will receive far different treatmentthan others, and none will receive the correct average time for whichthe installation was designed. If mechanical agitators are employed rtoplace the pulp mass in circulation sucient to inhibit stagnation,uniformity is again defeated, in that a portion of the bre will take adirect path from inlet to outlet in much less than calculated reactiontime, while other portions will circulate internally for much longerthan the calculated time. The result is both impaired quality of productand waste of chemicals.

It is therefore a more specic object of the present invention to providea simple and efficient method of effecting uniform and continuousbleaching, or similar treatment, of cellulose pulp or the like, withmaximum retention of pulp quality and at minimum cost.

Another specific object of my invention is to provide new and usefulapparatus for most eiectively treating cellulose pulp or the like incontinuous flow formation by the improved method.

Still another specific object of this invention is to provide animproved method of and apparatus for effectiing continuous and uniformtreatment of a fibre suspension, or any material having similar fluentcharacteristics, with a chemical reagent or other fluid medium.

These and other specific objects and advantages of the invention will beapparent from the following detailed description.

A clear conception of the several steps constituting my improved methodof treating brous material, and of one specific type of apparatus forcarrying on the method commercially, may be had by referring to thedrawing accompanying and forming a part of this specification, whereinlike reference characters designate the same or similar parts in thevarious views.

Fig. 1 is a central vertical section through a typical pulp bleachingreaction cell embodying my invention;

Fig. 2 is a. transverse horizontal section through the reaction cell ofFig. l, taken along the line 2 2; and

Fig. 3 is a perspective view of a fragment of the improved mixing rotorcomprising a part of the installation of Figs, l and 2.

The improved method constituting one portion of the present invention,comprises in general producing a constant displacement in one directionof a mass consisting of a mixture of fibrous material and a liquidtreating medium, by continuously introducing new mixture at one end ofthe mass and continuously discharging treated material from the oppositeend, and at periodic intervals along the path of displacement,re-distributing and intermixing the brous material and liquid in a planetransverse to direction of advancement, by means which do not affect thedisplacement of the mass and cause no movement of material outside thelimited zones of re-distribution.

Referring to the drawing showing one embodiment of apparatus forcarrying on the improved method which has been successfully applied tocommercial operation, a cylindrical casing 6, having a lower inlet 'Iand an upper outlet 8, provides an intervening vertically elongatedreaction chamber or space 9 of approximately circular cross-section.` Acentrifugal pump I0 or other suitable means is provided for constantlyforcing regulated quantities of fibrous material and treating agent intothe bottom of reaction space 9 through the inlet 'I. After passingupwardly through the entire vertical length of reaction space 9 by thedisplacement effected by the incoming mixture, the treated material isdischarged over dam 2| to outlet 8. The dimensions of the unit are madesuch that the volume of reaction space 9 in relation to the volume ofincoming mixture per unit of time, will cause any given horizontalstratum of material to rise through the reaction space in a period oftime equal to the optimum reaction cycle of the process being carried onin the unit. In commercial practice, units have been built with internaldiameters of from nine to sixteen feet, and a vertical height ofapproximately fty feet; while the rate of vertical displacement has beenon the order of four to six inches per minute.

Located centrally within the reaction space 9 is an upright shaft I Iwhich is rotatable in lower and upper bearings I2, I3 respectively, bymeans of an electric motor I4 which coacts with shaft II through speedreducing mechanism I5; and the shaft II carries a series of radial armsI8 forming rotor divisions spaced at suitable intervals along the shaftlength, and at approximately the level of dam 2l this shaft carries adischarge impeller I8. Each of the radial arms I6 has streamlinedtransverse cross-section, and is provided with two series of vanes I9and 2U, projecting vertically downward and upward from the lower andupper surfaces, respectively, and is also provided with an invertingblade I'I, projecting radially at an angle to the main arms. The vanesIS, 20 have increasing pitch or angularity proceeding toward the rotoraxis, and the lower vanes I9 are adapted to act upon the particlescomposing a shallow stratum of material immediately adjacent to thecorresponding arms I6 and to move such particles inwardly toward theaxis of shaft II; while the upper vanes 20 are similarly adapted to actupon adjacent particles, but are adapted to move such parti-clesoutwardly away from the axis of the shaft. The inverting blades II areshort as compared to arms I6, and have surfaces disposed at an obliqueangle relative to adjacent horizontal planes; and these blades I 'I areadapted to deflect particles which have been brought near the axis bythe action of lower vanes I9, upwardly into the Zone of action of uppervanes 20. In general the system of vanes associated with arms I6 is soformed and proportioned that it does not impart motion to the pulp massas a whole but acts only to move toward and away from the axis, suchfibrous particles as lie Within the zone of action of the vanes at anygiven time.

The pulp discharge impeller i8, which is 1ocated above the uppermost setof arms I6, has two radial arms which are curved longitudinally and Varyin depth at their leadingfaces, in such proportion that a uniformsection or transverse slice of the vertical pulp, column is swept overdam 2l and discharged to outlet 8 at each revolution of the rotor. Thelower end of casing 6 may also be provided with a drain 22 to facilitatefiushing and cleaning of the casing interior.

During normal operation of this improved reaction unit, for example inthe bleaching of paper pulp, the centrifugal pump l0 is operating tointroduce regulated quantities of pulp and bleaching agent into thelower extremity of the cell, and the material thus admitted rises slowlythrough the reaction chamber or space S during a predetermined period oftime, and is ultimately discharged by the impeller i8 over dam 2| andthrough the outlet 8. The motor lil should be operating to slowlyrevolve the shaft il and its rotor assemblies or divisions through speedreducing mechanism l5. As any given particle of pulp reaches the levelof the bottom of vanes i9 of any set of arms i6 it will be acted upon bythese vanes and moved inwardly toward the axis of the correspondingrotor assembly. Upon reaching a point near the shaft the particle willbe engaged by blade l1 and deflected upwardly into the Zone of action ofvanes 20, by which it will be moved outwardly away from the axis of therotor. The action hereinbefore described in relation to a discreteparticle, is in practice exerted upon a stratum of approximately twoinches vertical depth; and the rate of radial displacement inwardly andoutwardly produced by the action of the rotor is substantially in excessof the relative axial displacement of the stratum produced by the supplypump I0. Therefore the outermost upper vanes 20 tend to produce a slightpositive pressure by centrifugal force and the outermost lower vanes I9produce a slight negative pressure by cavitation, causing a portion ofthe material in each active stratum to return into the zone of lowerblades I9 and repeat the cycle of radial displacement. Under averageoperating conditions a typical particle will thus cycle some four oriive times before being axially displaced out of the zone of action ofeach rotor division. Between the upper boundary of the active zone ofone rotor assembly and the lower boundary of the active Zone of the nexthigher rotor division the pulp moves upwardly in a quiescent mass.

The action of the rotor assemblies or divisions upon the fibrousmaterial as herein described, performs two distinct functions, both ofwhich are indispensable to uniformity of treatment with minimum chemicalconsumption. First, each rotor division renders the stratum ofpulpslurry within its zone of action physically homogeneous, and thisperiodic combing of the advancing column overcomes the characteristictendency of a fibrous suspension to channel and stagnate as previouslydescribed in this specification. Thus all pulp receives equal retentiontime in the reaction chamber. Second, in each rotor division zone thefibres and treating agent are mutually 6 re-distributed and intermixed,thus enabling each bre to satisfy its optimum reactivity with a totalchemical supply to the system based on average fibre demand, whereas inthe absence of periodic redistribution the chemical dosage must be basedon maximum fibre demand, with resultant waste of chemicals anddegradation of the softer libres.

From the foregoing detailed description of the construction of myimproved apparatus and of the successive steps of the improved process,it should be apparent that the present invention provides a method ofand apparatus for most effectively bleaching or otherwise treatingfibrous material in a continuous manner and While the bre stock istravelling along a path in suspension. With the aid of relatively simpleand durable apparatus the improved method can be carried onautomatically. The present improved process and apparatus have provenhighly successful in actual commercial use, and have produced thedesired results with both a maximum chemical efficiency and extremelylow power consumption per unit of product. For example, an installationemploying a cell of sixteen feet diameter has handled a pulp productionof 650,000 pounds dry weight per twenty-four hours with an input of onlysixteen horsepower; whereas other attempted constructions have consumedmore than ten times as much power with a less satisfactory result.

It should be understood that it is not desired to limit this inventionto the exact details of construction of the apparatus, or to the precisesteps of the method, herein shown and described, for variousmodifications within the scope of the claims may occur to personsskilled in the art.

I claim:

1. The method of treating fibrous material resorting to the use of anaqueous medium giving fluidity to a slurry of the two and effective whenbrought into intimate association with the iibers of the material tochemically act upon the latter, and which comprises: giving travellingenergy to a continuous stream of the slurry and working the same in anupward direction at a sluggish rate of advance, and re-distributing andintermixing the fibers within the slurry by forcing a horizontal stratumof the upwardly moving stream to move in a direction which is generallyradial to the vertical stream axis and while maintaining the advancingstream adjoining said stratum in a state of comparative quiescence,there being a succession of said re-distributing and intermixingtreatments performed upon the stream at separated intervals within thelength of the streams upward travel.

2. The method of treating fibrous material resorting to the use of aliquid chemical giving fluidity to a slurry of the two and by intimate.association with the fibers of the material acting upon the latter, andwhich comprises: working a constantly advancing stream of the slurry ata sluggish rate of travel along a definite vertical path, andsuccessively at separated intervals along the prescribed pathre-distributing and intermixing the fibers within the slurry by causingthe stream in course of its traversal of limited transverse Zones to bediverted from said vertical path and move generally radial to the axialline of advancement, and while maintaining the advancing streamadjoining said limited transverse zones in a state of comparativequiescence.

3. The method of treating chemically-reactive fibers contained in aliquid body of the functioning chemical, and which comprises, givingtravelling energy to a continuous streamvof the mixture and working thesamel upwardly' at a sluggish rate of` travel along a denite path, andin course of the traversal oflongitudinally separated but immediatelyadjacent transverse zones lying within the length of said prescribedpath causing the stream to pursue routes of travel generally radial tothe central axis, giving to the stream a directional travel, radiallyconsidered, in one said zone opposite to that of the other andneighboring zone, and while maintaining the advancing stream in a stateof relative torpidity as it is brought to and as it thereafter leavessaid immediately adjacent transverse Zones.

4, The method of treating a chemically-reactive iibrous material, andwhich comprises the steps of giving travelling energy to a continuousstream of the material mixed with a liquid bodyv of the functioningchemical and working the same upwardly at a sluggish rate of travelalong;

a denite vertical pat-h; ige-distributing, and intermiXing the fibersand liquid chemical by causing the stream, in course of its traversal ofneighboring transverse zones lying within the length of said prescribedpath, to pursue generally radial routes of travel, giving to the streama directional travel in one said zone opposite to that of the other andneighboring zone; allowing the advancing stream to assume a state ofrelative torpidity following said re-distributing and intermixing; andrepeating said re-distributing and intermixing of the fibers and liquidchemical in course of traversal by the stream of other neigh- 8. boringtransverse Zones lying within the length of said prescribed path andlongitudinally removed from the iirst-mentioned zones.

5-. The method of claim 4 in which the directional travel of the streamin its radial movement within the said neighboring transverse zones isrst inwardly toward the central axis and thereafter outwardly away fromsaid central axis, being made to accelerate its travelling speed inmoving toward and decelerate its travelling speed in moving from saidcentral axis.

RAYMOND P. HILL.

REFERENCES CITED- l5; The following references are of record in the fileof this patent:

UNITED STATES PATENTS Number Name Date 211,772 OHarra Jan. 28, 1879525,095 Detwiler Aug. 28, 1894 989,126 Currie Apr. 11, 1911 1,231,499Crossley June 26, 1917 1,768,955 Johnson July 1, 1930 245 1,915,812Wollenberg June 27, 1933 1,943,031' Meissner Jan. 9, 1934 1,976,955MacLean Oct. 16, 1934 FOREIGN PATENTS soi Number Country Date 517,797Great Britain Feb. 8, 1940 7,872 Great Britain 1913

